The only sub HSU has in that price range is the STF1 which is $299.
Here are some budget subs which have garnered a very good reputation:
BIC Venturi V1220 $224
Dayton ES10 Elite $230
Dayton SUB-120 HT $155
Dayton SUB-100 HT $131
BIC Acoustech H-100 $250
Premier Acoustic PA-120 $230
TSC T250 Subwoofer $250
All the prices I took from
parts-express.com or
sounddistributors.com, except for the Speaker Company one, obviously. There may be DIY kits from which you can get a much better performing sub for the same money, but you have to ask around about that. I'm not sure if any of those kits are under 300.
Here is one for $320, I don't know how good it is though. I would guess it could outperform the ones listed above, going by the specs.